Embodiments of the present invention relate to jitter and noise, and more particularly, to instruments, systems and methods for deconvolving, synthesizing and analyzing jitter and noise.
Jitter and noise are signal impairments that cause signal quality problems in high-frequency signals. Jitter and noise may be caused by various types of sources, such as electromagnetic interference, crosstalk, data-dependent effects, random sources, and so forth. In general, jitter may be identified as the “horizontal” displacement of various aspects of pulses in a high-frequency signal or waveform, and noise can be thought of as the “vertical” displacement. Jitter may be identified on the horizontal axis of an oscilloscope (typically measured in units of time), while noise may be identified on the vertical axis of an oscilloscope (typically measured in units of voltage).
More specifically, the term jitter refers to the horizontal displacement from an ideal position of various aspects of pulses of a signal or waveform, such as the displacement of various aspects of pulses of a signal or waveform within the time domain, phase timing, or the width of the pulses themselves. The term noise refers to the vertical displacement of various aspects of pulses of a signal or waveform, such as amplitude error in the signal or other vertical noise effects.
Crosstalk between adjacent channels of high-speed interconnects significantly affects the performance of serial links. The presence of crosstalk impairs the accuracy of the current jitter and noise measurement methodologies, resulting in reductions to the design margins for the high-speed link devices at a high cost to manufacturers.
The analytical models used by the test and measurement industry have been evolving to allow for more accurate estimation of jitter and noise behavior at higher bit error rates (BER). Complementing the jitter analysis with noise analysis proved to be a more accurate predictor of the BER than jitter alone, such an approach lending itself to root-cause problem identification.
Jitter and noise can be “decomposed” (e.g., separated) into various components in order to aid in the analysis of the total impairment of communication link or an associated system (e.g., transmitter, receiver, transmitter and receiver pair, electronic device or component, etc.) using a test and measurement instrument such as an oscilloscope. Conventional approaches for decomposing jitter include separating deterministic jitter (DJ) from random jitter (RJ), and then “reassembling” (e.g., synthesizing or convolving) the jitter components for analysis of the total jitter at a specific bit error rate (BER), sometimes referred to as TJ@BER.
Particularly in the presence of crosstalk, uncorrelated jitter or noise comprises both bounded and unbounded components. In one decomposition approach, as disclosed in commonly-owned U.S. Pat. No. 7,522,661, jitter or noise is decomposed into correlated and uncorrelated components. The uncorrelated component can be further decomposed—e.g. by spectral separation—into periodic BUJ (P-BUJ or PJ) and, as disclosed in commonly-owned U.S. application Ser. No. 13/081,369, bounded non-periodic jitter.
Crosstalk is mostly a source of bounded noise, and pertinent to this work it is largely uncorrelated to the data stream in the link under test. In the performance assessment of serial devices, the effects of crosstalk on jitter need to be better characterized. One difficulty for the current spectral analysis based tools arises when the crosstalk spectrum becomes broadband, such as for aggressors with long patterns, non-repetitive serial traffic, synchronous or asynchronous to the victim under test. The resulting spectral flooding leads to the lifting of the noise and jitter floors, thereby rendering the crosstalk components indistinguishable from the residual random elements. What is needed is an enhanced ability to quantify the effects of crosstalk in the victim channel (i.e., the channel being affected by the crosstalk).
In addition, the division between periodic jitter and non-periodic jitter can be imperfect because of a variety of observation-time-related or computational factors, which can lead to partial correlation between the periodic and non-periodic categorizations, which in turn can lead to inaccuracies when the jitter is subsequently mathematically convolved because of the presence of partially correlated distributions, which itself in turn can result in inaccurate (i.e., overly pessimistic) estimates of TJ@BER. Conventional methods are not capable of supplying enough information about the jitter to discern this possible partial correlation, or the degree to which the components might be correlated.
While the decomposition of jitter and noise are important for understanding the root cause of signal impairment, there is a need for those designing and measuring fast communication links to be able to accurately predict the BER level, and the immediately underlying BER limiting factors such as the total jitter (TJ@BER), the total noise (TN@BER), and the whole BER eye and/or contours. There remains a need for providing improved methods and systems for accurately accomplishing such calculations using an oscilloscope by algorithmic synthesis from components found in the decomposition of the jitter or noise.
The foregoing and other features and advantages of the inventive concepts will become more readily apparent from the following detailed description of the example embodiments, which proceeds with reference to the accompanying drawings.